This invention relates to a screw joint coupling for oil pipes.
Since the screw joint coupling for oil pipes is used under high pressure, high load and other severe conditions, many studies have been made of various kinds of coupling structures.
Known screw couplings are so-called metal sealing system. The metal seal is prepared with a sealing portion shaped in taper at the end point of a metal screw (smooth outer circumferential part) and a sealing portion shaped in taper at an inner part of a female screw (smooth inner circumferential part), and provided by butting the end point of the male screw and the end point of a stopper formed at an inner side of the female screw; or said sealing part of the male screw is formed in convex in an axial direction.
This kind of coupling does not always provide an adequate air-tightness, so that it can cause stress corrosion cracking. Moreover, with respect to the structure of this coupling, in order to improve the sealing property a determined inclining angle is given to the end point of the male screw and that of the stopper, to thereby force up the male screw so that the sealing portion of the male screw is urged toward the female screw. But because very strong force is exerted on the stopper, bending moment or wedging effect are applied to the root of the stopper, and this portion is consequently affected by stress corrosion.
The present inventon has been developed in view of the above mentioned problems encountered by the prior art.
It is an object of the invention to provide a structure of a joint coupling for oil pipes which has excellent strength and sealing properties without bringing about problems such as stress corrosion cracking.
It is another object of the invention to provide a structure of a joint coupling for oil pipes which has an excellent sealing property without designing an inclining angle in the end point of the male screw and the end part of a stoper formed at the inner part of the joint coupling, whereby the stress corrosion to be caused by said inclining angle is thereby avoided.
It is a further object of the invention to provide a structure of a joint coupling for oil pipes which maintains a high quality sealing property in spite of repeated of screwings and unscrewings of the coupling.